The present invention relates to diode lasers and more particularly, a configuration of and a method for optical beam-shaping of diode laser bars to produce an optical beam of high power and brightness, allowing for efficient coupling of the optical diode laser bar output into an optical fiber.
High power solid state lasers and particularly high power fiber lasers generally depend on the availability of optical pump beams with high optical power and brightness, i.e., the required pump power needs to be made available in as small a space as possible. Semiconductor diode laser arrays are firmly established as the main source of high power optical beams, i.e., 1 watt and greater including ultra-high powers of greater than 10 watts. With current semiconductor technology, a power approaching 40 kW can be produced by a stacked diode array measuring only 5xc3x9710 cm in size (R. J. Beach et al., Laser Focus World, December 2001).
The stacked diode array typically consists of individual diode bars of around 1 cm in-length, which in turn incorporate 10-50 individual 100-200 xcexcm long emitters spaced 0.1-1.0 mm apart. Generally, the total length of the individual emitters comprises only a fraction of the total bar length. The ratio N*li/L, where N is the number of individual emitters, each of length li, and L is the total length of the diode bar, is referred to as fill factor. In a diode stack the diode bars are typically separated by 1-2 mm and arranged in tiles of 10-20. Each bar typically produces a cw (continuous wave) power up to 100 W. Thus for 40 kW of power, 400 bars are needed.
A laser beam originating from one individual emitter typically has a divergence of 10xc2x0xc3x9750xc2x0. The small divergence beam is in the plane of the x axis and the high divergence beam is in the plane of the y axis. These axes are sometimes referred to as slow and fast axis respectively. The fast axis beam is generally diffraction limited and can be collimated to within a fraction of the diffraction limit with a cylindrical lens aligned parallel to the slow axis of the single emitter. The slow axis beam is typically 10-20 times diffraction limited.
The brightness B of the optical beam from an individual emitter can be calculated as B=power/(emitting areaxc3x97angular divergencexxc3x97angular divergencey), where angular divergencex,y represents the angular divergence along the x and y axes respectively. For an individual emitter of dimensions 1xc3x97100 xcexcm operating at a power of 4 W a brightness of B=27 MW/cm2 is obtained.
In contrast, the brightness of a diode bar with dimensions 1 xcexcmxc3x971 cm operating at a power level of 100 W is only about 6.6 MW/cm2, whereas the brightness of a diode stack as described above operating at a power of 40 kW is only of the order of 5 kW/cm2. High brightness diode bars are generally manufactured by maximizing the fill factor; fill factors as high as 80-90% are commonly used to reach bar power levels of 100 W or more.
For many applications, such as fiber coupling, focusing of the diode beams into as small a spot as possible is required, where ideally the dimension of the focused spot should be the same along both axes of the diode beam. The diode bar from the example above emitting a power P=100 W can for example be coupled into a fiber with a diameter of 4 mm and a numerical aperture NA=0.22. However, the fiber coupled brightness B thus reduces to 4 kW/cm2, where the brightness B obtained from a fiber with core area A is calculated as B≈P/(A*xcfx80NA2).
The brightness of diode laser beams has traditionally been increased by the implementation of beam-shaping optics, ie., by optically combining the individual emitter beams from the diode array to generate a single optical beam which can be efficiently coupled into an optical fiber (See, e.g. U.S. Pat. No. 5,168,401 of Endriz, hereinafter Endriz ""401).
Optical beam-shaping is possible by a variety of means. A first class of methods uses optical beam rotation of each emitter to rotate the beam by 90xc2x0, where the direction of the emitter beam is further deflected by around 90xc2x0after reflection from at least two reflecting surfaces. The Endriz ""401 patent describes such an example. Further examples of such a method are described in U.S. Pat. No. 5,418,880 of Lewis et al. and U.S. Pat. No. 6,044,096 of Wolak, et al.
A second class of methods is based on optical beam rotation using beam-rotating prisms such as the Abbe-Konig prism as disclosed in U.S. Pat. No. 5,243,619 of Albers et al. The advantage of this design is that it avoids a 90xc2x0 deflection of the beam direction such that only a small displacement in the propagation direction results.
A third class of methods is based on beam deflection in a set of multi-facetted mirrors or prisms. In these methods a first multi-facetted optical structure deflects the beam to obtain some beam spacing in the y direction, while a second multi-facetted optical structure deflects the beams to overlay the beams along the x direction (see, e.g., U.S. Pat. No. 5,887,096 of Du et al., U.S. Pat. No. 6,151,168 of Goering et al. and U.S. Pat. No. 5,987,794 to Ullmann et al.). Such systems do not require beam rotation optics, but generally employ a beam deflection along the propagation direction, relying on the manufacturing of expensive high precision multi-facet bulk optics. Though the beam-shaping device described by Ulmann et al. can work for high fill factor bars and diode arrays, the optical path lengths of individual beams through the beam-shaping optic are generally different, thereby limiting the focussability of the resulting beam. Moreover, it is difficult to implement this method with bars tightly stacked in one dimension.
The function of beam deflection and overlay can also be accomplished in one single optical element as disclosed by U.S. Pat. No. 5,825,551 of Neilson, et al. A limitation of the approach taken by Neilson et al. is the variation between the optical path lengths of each individual emitter beam through the beam-shaping optic, which in-turn limits the focussability of the resulting beam. Moreover, the adaptation of the technique by Neilson et al. to fiber coupling of diode bar stacks requires two such sets of beam-shaping optics, which is expensive to implement. Note that the first three classes of beam-shaping optics use non-focussing optics.
A fourth class of beam-shaping methods is based on beam rotation in an array of imaging transmissive optical elements that comprises at least one spherical cylindrical surface or a toroidal surface with two different curvatures along two orthogonal axes. (Such an array is described by Lissotschenko et al., German Patent No. DE 19920293 and U.S. Patent Publication No. 2002/0015558) as depicted in FIG. 1. As shown, beam-rotation assembly 100 comprises four pairs of spherical cylindrical lenses 101-104, which beam-rotate four individual beamlets 105-108 emitted from a diode bar with a slow axis represented by line 109. FIG. 1 represents only the front surfaces of the beam-rotating cylindrical lens pairs; the back surfaces of the beam-rotating cylindrical lens pairs are positioned directly behind the front surfaces and are not visible. In FIG. 1 the individual beamlets 105-108 are depicted as squares to represent the approximate extension of the beamlets after collimation of the fast axis. The axes of the cylindrical lenses are aligned at an angle of 45xc2x0 with respect to the slow axis 109 of the diode bar. To avoid beam-clipping in the cylindrical lenses, the beamlets need to be located centrally within the cylindrical lenses. It can be seen from FIG. 1 that the separation of the individual emitters along the slow axis must be twice their length (for square beamlets) to fit the beam through the beam-shaping optical element. Hence the theoretical maximum allowable diode fill factor is 50%, practical implementations resulting in an actual maximum fill factor of about 40% because of beam-divergence, which restricts the effective aperture of each beam-shaping optical element. Another limitation of the beam-rotating device described by Lissotschenko et al. is that optical aberrations remain uncompensated, hence the effective use of the optical beam-rotating element is limited to small NA optics, increasing the required length of the toroidal cylindrical beam-shaping optic and further limiting the aperture of each toroidal cylindrical lens, resulting in even lower available diode bar fill factors.
Generally all techniques described so far incorporate separately formed optical elements, i.e., are not monolithic, and therefore require complex alignment procedures and resultant high manufacturing costs. Moreover, the application of optical coating(s) is also difficult in these devices which can limit the optical throughput.
A beam-shaping device which can overcome these problems is described in U.S. patent application Ser. No. 10/085,620, filed Mar. 1, 2002, and incorporated herein in its entirety. That device is based on the use of for example planar graded index (grin) lenses or Fresnel lenses with a magnification of M=xe2x88x921 to achieve beam-rotation by 90xc2x0 for all the individual emitter elements in a diode bar. Both planar grin lenses and Fresnel lenses allow for essentially aberration free imaging, hence they are not restricted to low numerical apertures. Moreover, monolithic or quasi monolithic beam-shaping elements can be constructed with both planar grin lenses and Fresnel lenses, allowing for effective fiber coupling of diode bars from very compact assemblies.
The present invention is directed to an optical arrangement allowing efficient coupling of a high-fill factor diode bar into an optical fiber. The device uses low aberration, high NA imaging elements that provide a magnification of M=xe2x88x921, resulting in beam inversion. Moreover, the individual optical elements are produced in a rhombic form, allowing for side to side alignment of each beam-rotating imaging element without any unused dead space.
The beam inversion optic is based on arrays of grin lenses, cylindrical Fresnel lenses or aspheric cylindrical lenses. The graded index optic is preferably planar, where beam inversion is obtained by aligning the lines of equal refractive index at an angle of approximately xc2x145xc2x0 (as used herein, the xe2x80x9cxc2x1xe2x80x9d sign refers to an angular displacement of the specified amount in both positive and negative rotational directions and not to a range of angular displacements) with respect to the slow axis of the individual emitters. In the case of cylindrical Fresnel lenses, the lines of equal phase retardation are equally aligned at an angle of approximately xc2x145xc2x0 with respect to the slow axis of the individual emitters. In the case of aspheric cylindrical optics, the cylinder axis is also aligned at an angle of approximately xc2x145xc2x0 with respect to the slow axis of the individual emitters.
Beam inversion is further facilitated by first collimating the fast axis of the individual emitters with a single cylindrical lens aligned parallel to the slow axis of the diode bar. Highly integrated monolithic beam inversion optical systems are also possible. The monolithic beam inversion optical system comprises an integrated fast axis collimation element in conjunction with a beam rotation element. An additional slow axis collimating element can also be incorporated in a monolithic fashion. The beam inversion optic can further be used for beam-shaping of the output of individual emitters, facilitating efficient coupling of the output into optical fibers.
Ultra-high power optical beams can be obtained by implementing the beam-inverting optic with stacks of diode bars, where arrays of beam-shaping optics can be readily implemented. Additional beam-cutting optics can be incorporated to facilitate coupling into a required fiber size.
Alternatively, ultra-high power optical beams can be obtained by combining the output of individual fiber-coupled diode bars into an optical fiber bundle that is operated in conjunction with an efficient heat sink; for optimum brightness conservation the fibers can be designed with a rectangular cross section.